1. Field of the Invention
This invention relates to an information data processing apparatus for transmitting information data, such as image data or the like, and to an encoding apparatus for encoding the information data.
2. Description of the Related Art
Digital SNG (satellite news gathering) is known as a typical method for transmitting a digital image to a remote location. In the digital SNG, by controlling the amount of generated information in accordance with a preset necessary picture-quality mode and generating a plurality of carriers in a single transponder, a plurality of stations can be simultaneously operated.
A description will now be provided of a conventional digital SNG transmission method.
FIG. 1 is a block diagram illustrating a conventional digital SNG transmission unit.
In FIG. 1, a digital image signal input via an input terminal 71 is subjected to data compression by an image-data encoding unit 72 according to an encoding method such as represented by MPEG (Moving Picture Experts Group) 2. A header, an error-correcting code and the like are added to the signal subjected to data compression by a multiplexing unit 73, and the resultant signal is transmitted to a modulation unit 74.
The output rate at this stage differs depending on which preset mode has been selected from among three modes. For example, the information rate is high in a high picture quality (HQ) mode in which high quality is required for the material of the program. The information rate is rather low in a standard picture quality (NQ) mode in which it is only necessary to understand the contents of news. The information rate is further compressed to a low value in a mode in which high picture quality is not required, such as a secondary distribution picture quality (SQ) mode.
The modulation unit 74 performs digital modulation of a carrier by the input data. The modulated wave is subjected to frequency conversion into a predetermined frequency by a frequency conversion unit 75, and is then amplified by a high-frequency amplification unit 76, and the amplified signal is output from an antenna 77. In order to effectively use a transponder, the output frequency is set to a different frequency depending on the mode.
This operation will be described with reference to FIG. 2.
FIG. 2 is a diagram illustrating the arrangement of the frequencies of transmission signals.
In the digital SNG transmission unit having the high picture quality mode, the standard picture quality mode and the secondary distribution picture quality mode, the central frequency when transmitting a signal in the high picture quality mode is F3, and a frequency F1 is selected as the central frequency for the secondary distribution picture quality mode. The central frequency when transmitting the signal in the standard picture quality mode is selected from among F2, F4 and F5 depending on the state of the use of bands by other stations.
As described above, in order to effectively use bands, it is necessary to optimize the relationship between the occupied bandwidth and the central frequency of the carrier in each mode.
In the above-described digital SNG system, the central frequencies of carriers are set so that images for respective programs having different picture quality levels can be effectively transmitted. However, since the frequencies are not arranged symmetrically with respect to the central frequency F0 of the band allocated for images, if one wave is output within the band, the degree of freedom of the transmission frequencies of other stations is greatly restricted.
In a conventional image data encoding method, image data is divided into a plurality of blocks, and encoding is performed for each of the blocks. The amount of codes obtained as a result of encoding greatly differs depending on the contents of the image. Hence, in order to obtain a constant transmission rate, processing for maintaining the transmission rate at a constant value, in which encoded image data is stored in a buffer memory and the quantization coefficient is changed using the degree of occupation of the buffer memory, is performed.
Accordingly, in the conventional encoding method, even when a block having a high-frequency component in the contents of the image is adjacent to a block having a low-frequency component in the contents of the image, the same level of quantization is performed. As a result, encoding noise which is hardly recognizable in the block having the high-frequency component is clearly observed in the block having the low-frequency component, thereby degrading the quality of the image.